Fuel cells convert gaseous fuels (such as hydrogen, natural gas and gasified coal) via an electrochemical process directly into electricity. A fuel cell continuously produces power when supplied with fuel and oxidant, normally air. A typical fuel cell consists of an electrolyte (ionic conductor, H+, O2−, CO32− etc.) in contact with two electrodes (mainly electronic conductors). On shorting the cell through an external load, fuel oxidises at the anode resulting in the release of electrons which flow through the external load and reduce oxygen at the cathode. The charge flow in the external circuit is balanced by ionic current flows within the electrolyte. Thus, at the cathode oxygen from the air or other oxidant is dissociated and converted to oxygen ions which migrate through the electrolyte membrane and react with the fuel at the anode/electrolyte interface. The voltage from a single cell under load conditions is in the vicinity of 0.6 to 1.0 V DC and current densities in the range 100 to 1000 mAcm−2 can be achieved.
Several different types of fuel cells have been proposed. Amongst these, the solid oxide fuel cell (SOFC) is regarded as the most efficient and versatile power generation system, in particular for dispersed power generation, with low pollution, high efficiency, high power density and fuel flexibility. SOFC's operate at elevated temperatures, for example 700-1000° C. Other fuel cells which operate at elevated temperatures include the molten carbonate fuel cell requiring a minimum temperature of 650° C. However, SOFC's are the primary interest for the invention and further discussion herein will be mainly directed to these without intending to be limited in any way.
Numerous SOFC configurations are under development, including the tubular, the monolithic and the planar design. The planar or flat plate design is the most widely investigated. Single planar SOFC's are connected via interconnects or gas separators to form multi-cell units, sometimes termed fuel cell stacks. Gas flow paths are provided between the gas separators and respective electrodes, for example by providing gas flow channels in the gas separators. In a fuel cell stack the components—electrolyte/electrode laminates and gas separator plates—are fabricated individually and then stacked together. With this arrangement, external and internal co-flow, counter-flow and cross-flow manifolding options are possible for the gaseous fuel and oxidant.
Traditionally hydrogen, usually moistened with steam, has been used as a fuel cell fuel. However, in order to be economically viable the fuel must be as cheap as possible. One relatively cheap source of hydrogen is natural gas, primarily methane with a small proportion of heavy hydrocarbons (C2+). Natural gas is commonly converted to hydrogen in a steam reforming reaction, but the reaction is endothermic and, because of the stability of methane, requires a reforming temperature of at least about 650° C. for substantial conversion and a higher temperature for complete conversion. While high temperature fuel cell systems produce heat which must be removed, heat exchangers capable of transferring thermal energy at the required level of at least about 650° C. from the fuel cells to a steam reformer are expensive. Thus, hydrogen produced by steam reforming natural gas may not be a cheap source of fuel.
One proposal of a fuel cell electricity generation process in which a hydrocarbon fuel is converted to a fuel cell fuel stream including hydrogen in a steam pre-reformer is disclosed in EP-A-0435724. The temperature in the pre-reformer is described as 700 to 850° C. with a resultant product-gas composition of 65-80 vol % H2, 5-20 vol % CO, and 5-25 vol % CO2.
Another such proposal is disclosed in U.S. Pat. No. 5,302,470 in which the steam pre-reforming reaction is said to be carried out under similar conditions to those of known steam reforming reactions: for example, an inlet temperature of about 450 to 650° C., an outlet temperature of about 650 to 900° C., and a pressure of about 0 to 10 kg/cm2.G to produce a fuel cell fuel stream which is composed mainly of hydrogen and is fed to the fuel cell anode via a carbon monoxide shift converter.
Hydrocarbon fuels suggested for use in the above two proposals include, in addition to natural gas, methanol, kerosene, naphtha, LPG and town gas.
It has been proposed to alleviate the aforementioned problem of the cost of substantially complete steam pre-reforming of methane by using natural gas as a fuel source for a high temperature planar fuel cell stack and subjecting the natural gas to steam reforming within the stack, at a temperature of at least about 650° C., using catalytically active anodes. However, this arrangement can lead to carbon disposition problems on the anode from C2+ hydrocarbons and is not suited to other higher hydrocarbon fuels for this reason. Furthermore, given the endothermic nature of the methane steam reforming reaction, too much methane in the fuel stream can lead to excessive cooling of the fuel cell stack. To alleviate this problem the fuel stream has been restricted to a maximum of about 25% methane (on a wet basis) with the natural gas being subjected to partial steam pre-reforming at elevated temperatures approaching 700° C. upstream of the fuel cell stack.
Another process for producing electricity in a fuel cell from hydrocarbon fuels such as gasified coal, natural gas, propane, naphtha or other light hydrocarbons, kerosene, diesel or fuel oil is described in EP-A-0673074. As described in that specification, the process involves steam pre-reforming approximately 5 to 20% of the hydrocarbon fuel at a temperature of at least 500° C. after start-up to convert ethane and higher hydrocarbons in that fraction to methane, hydrogen and oxides of carbon and to achieve a measure of methane pre-reforming in that fraction to oxides of carbon and hydrogen. Steam pre-reforming at this lower temperature alleviates carbon deposition in the pre-reformer. The hydrocarbon fuel with the steam pre-reformed fraction is then supplied to fuel inlet passages of the fuel cell stack which are coated with or contain a catalyst for steam reforming of the methane and remaining hydrocarbon fuel at 700-800° C. into hydrogen and oxides of carbon which are supplied to the anodes in the fuel cell stack.
Indirect internal steam reforming of the remaining hydrocarbon fuel within the fuel inlet passages is said to allow the use of reforming catalysts within the fuel inlet passages which are less likely to produce coking or carbon deposits from the internal steam reforming of the higher hydrocarbons than nickel cermet anodes. It is believed that steam pre-reforming of the hydrocarbon fuel in the described temperature range is restricted to 5 to 20% of the fuel in order to relatively increase the level of hydrogen in the fuel stream to the fuel cell stack and thereby alleviate carbon deposition when the fuel is internally reformed in the stack.
An alternative approach to providing a fuel stream for a fuel cell in which the proportion of methane derived from a higher carbon (C2+) hydrocarbon fuel is increased is disclosed in our International Patent Application No PCT/AU00/00974 filed 16 Aug. 2000, the contents of which are incorporated herein by reference. In this proposal all the fuel is reacted with steam in a steam pre-reformer at a temperature in the pre-reformer of no greater than 500° C. to produce a fuel stream including hydrogen and no less than about 20% by volume methane (measured on a wet basis). The fuel stream is reacted at the anode of the fuel cell to produce electricity when an oxidant such as air is reacted at the fuel cell cathode.
By this proposal, any of a wide range of higher hydrocarbon fuels may be used, and the lower pre-reforming temperature of no greater than 500° C. not only results in a greater proportion of methane being produced but also enables a simpler and therefore cheaper pre-reformer system to be adopted.
It has been found advantageous to increase the proportion of methane in the fuel stream to a high temperature fuel cell in which the methane is internally reformed on the anode because consumption of the heat released from the exothermic fuel cell reaction by the endothermic steam/methane internal reforming reaction leads to better thermal management of the fuel cell. In turn this provides improved fuel cell efficiency because of reduced parasitic losses associated with cooling strategies otherwise required for the fuel cell. Any additional methane content in the fuel stream replacing hydrogen means more internal reforming and therefore lessened requirement for cell cooling which is normally achieved by flowing excess air through the cathode side of the fuel cell. However, a disadvantage of the proposal in PCT/AU00/00974 is that it is seeking to balance the production of methane in the fuel stream against the desire to pre-reform all the higher hydrocarbons in the initial fuel. Temperatures towards the upper limit of the range (no greater than 500° C.) defined in that application, or higher, may be required for full conversion of the higher hydrocarbons (because of practical limitations imposed by reaction kinetics and/or catalyst effectiveness), but thermodynamics require a temperature lower than this to optimize the proportion of methane in the fuel stream. The present invention seeks to alleviate this disadvantage.